The present invention relates to a zoom lens system suited to incorporation in a digital camera or an information terminal such as a mobile phone equipped with a digital camera function.
Digital cameras are known where an image pickup device such as a CCD or a CMOS produces digital data for an image formed by a lens or lenses. Information terminals such as mobile phones in which a digital camera is incorporated are also known.
There is demand for a lightweight, compact zoom lens system for use as a zoom lens installed in a compact digital camera or a mobile information terminal. One method of reducing the weight of a lens system is to use plastic lenses in the lens system.
Japanese Laid-Open Patent Publication No. 2002-72091 discloses a zoom lens system that includes plastic lenses and is composed of a total of eight or nine lenses in three lens groups whose refractive powers are respectively negative, positive, and positive or negative, positive, and negative in that order from the object (subject) side. In this publication, a zoom lens system with a total of three plastic lenses composed of one lens in each group is also disclosed as the example with the maximum number of plastic lenses.
Japanese Laid-Open Patent Publication No. 2002-72093 similarly discloses a zoom lens system that includes plastic lenses and is composed of a total of eight or nine lenses in three lens groups whose refractive powers are respectively negative, positive, and positive or negative, positive, and negative in that order from the object side. In this publication, a zoom lens system with a total of four plastic lenses composed of two lenses in the first lens group and one lens in each of the second and third groups is disclosed as the example with the maximum number of plastic lenses.
To further reduce the weight of a zoom lens system and make the system more compact, it is preferable to increase the proportion of the number of plastic lenses to the total number of lenses and to further reduce the total number of lenses that construct the lens system. However, since the thermal expansion coefficient of plastic lenses is highly dependent on temperature, it is difficult to utilize plastic lenses for lenses that have a high refractive power in a digital camera or an information terminal that will be used in various temperature conditions. Even in the publications described above, the proportion of number of plastic lenses is half or less of the total number of lenses.
Also, since the surface strength of plastic lenses is low compared to glass lenses, it is difficult to use a plastic lens for the closest lens to the object side that is exposed to the outside. In this way, it is difficult to increase the proportion of the number of plastic lenses used in a zoom lens system.
To realize a compact, lightweight zoom lens system, it is also important to make the driving mechanism for moving the lens groups during zooming (variation in magnification) more compact. In Japanese Laid-Open Patent Publication No. 2002-72093, a larger proportion of plastic lenses is used compared to Japanese Laid-Open Patent Publication No. 2002-72091. However, this zoom lens system is designed with a premise of all three lens groups being moved during zooming, which makes the driving mechanism complicated and means such system is unsuited to incorporation in a mobile apparatus.
U.S. Pat. No. 6,308,011 discloses a zoom lens constructed of a total of seven or eight lenses in three units whose refractive powers are negative, positive, and positive in that order from the object side. In this document, 4 types of the second lens unit are disclosed. Four types are, in order from the object side: (a1) a single positive lens; (a2) a single positive lens and a single negative lens; (a3) a positive lens, a negative lens, and a positive lens; and (a4) a positive lens, a positive lens, a negative lens, and a positive lens. Plastic lenses are fundamentally not used in this second lens unit. The document also states that it is preferable to include an aspherical surface in the construction (a4) to reduce the size of the second lens unit. The document also discloses that the arrangement (a3) is most preferable, with a combination of a bi-convex lens, a bi-concave lens, and a bi-convex lens in that order from the subject (i.e., a so-called triplet) being preferably used as the second lens unit.